There is an important commercial need to obtain a color proof that will accurately represent at least the details and color tone scale of the image before a printing press run is made. In many cases, it is also desirable that the color proof accurately represents the image quality and halftone pattern of the prints obtained on the printing press. In the sequence of operations necessary to produce an ink-printed, full-color picture, a proof is also required to check the accuracy of the color separation data from which the final three or more printing plates or cylinders are made.
In general, the image in a color proof is formed by transferring a colorant (e.g. a dye, pigment, metallic or white and opaque spot colorant) from a donor element to a receptor element under the influence of energy from an energy source such as a thermal printhead or a laser. This transfer can occur via mass transfer or dye transfer.
In a mass transfer system, the majority of the material on the donor element (e.g., colorant, binder, and additives) is transferred to the receptor element. Typically, this can occur either by a melt mechanism or by an ablation mechanism. In a melt mechanism, the donor material is softened or melted. This softened or molten material then flows across to the receptor. This is typically the mechanism at work in a thermally induced wax transfer system. In an ablation mechanism, gases are typically generated that explosively propel the donor material across to the receptor. This results from at least partially volatilizing the binder or other additives in and/or under a layer of the donor material to generate propulsive forces to propel the colorant toward the receptor.
The image in a color proof formed from a mass transfer system is typically a half tone image. In a system that forms half tone images, the transfer gives a bi-level image in which either zero or a predetermined density level is transferred in the form of discrete dots (i.e., pixels). These dots can be randomly or regularly spaced per unit area, but are normally too small to be resolved by the naked eye. Thus, the perceived optical density in a half tone image is controlled by the size and the number of discrete dots per unit area. The smaller the fraction of a unit area covered by the dots, the less dense the image will appear to an observer.
In a dye transfer system, only the colorant is transferred from the donor to the receptor. Thus, the colorant is transferred without binder or other additives. This can occur either by a diffusion mechanism or a sublimation mechanism.
The image in a color proof formed from a dye transfer system is typically a continuous tone (i.e., contone) image. In a continuous tone or contone image, the perceived optical density is a function of the quantity of colorant per pixel, higher densities being obtained by transferring greater amounts of colorant. To emulate half tone images using a thermal dye transfer system, a laser beam can be modulated by electronic signals which are representative of the shape and color of the original image to heat and ultimately volatilize dye only in those areas where the dye is required on the receptor element to reconstruct the color of the original object. Further details of this process are disclosed in GB Publication No. 2,083,726 (3M). U.S. Pat. No. 4,876,235 (DeBoer) and U.S. Pat. No. 5,017,547 (De Boer) also disclose a thermal dye transfer system in which the perceived optical density is obtained by controlling the tonal gradation or thickness (density) of the colorant per pixel. In this system, the receptor element also includes spacer beads to prevent contact between the donor element and receptor element. This allows for the dye to diffuse or sublime across to the receptor element without the binder.
The shape and/or definition of the dots can effect the quality of the image. For example, dots with more well defined and sharper edges will provide images with more reproducible and accurate colors. The shape and/or definition of the dots are typically controlled by the mechanism of transfer of the image from the donor element to the receptor element. For example, as a result of the propulsive forces in an ablation system, there is a tendency for the colorant to “scatter” and produce less well defined dots made of many fragments. Attempts have been made to produce more well defined dots using an ablation system, such as those described in U.S. Pat. No. 5,156,938 (Foley) and U.S. Pat. No. 5,171,650 (Ellis), but such systems do not produce contract-quality images.
In contrast to ablation systems, melt systems can in principle form more well-defined dots and sharper edges to achieve more reproducible and accurate colors. Such systems, however, are not free of disadvantages. Many of the known laser-induced melt transfer systems employ one or more waxes as binder materials. The use of a wax results in a transfer layer that melts sharply to a highly fluid state at moderately elevated temperatures thus resulting in higher sensitivity. At the same time, however, melt systems are prone to image spread as a result of wicking or uncontrolled flow of the molten transfer material. Furthermore, because the laser absorber is normally transferred along with the desired colorant, the final image may lack the accuracy of color rendition required for high quality proofing purposes. Attempts have also been made to increase the sensitivity of the proofing systems by adding plasticizers (U.S. Pat. No. 5,401,606 (Reardon)), which lowers the melt viscosity and increases the flow, however, the plasticizers soften the films such that they become receptive to impressions and blocking.
The ability to image using a laser imaging source introduces significant advantages. For imaging by means of laser-induced transfer, the donor element typically includes a support bearing, in one or more coated layers, an absorber for the laser radiation, a transferable colorant, and one or more binder materials. When the donor element is placed in contact with a suitable receptor and subjected to a pattern of laser irradiation, absorption of the laser radiation causes rapid build-up of heat within the donor element, sufficient to cause transfer of colorant to the receptor in irradiated areas. By repeating the transfer process using different donor elements and the same receptor, it is possible to superimpose several monochrome images on a common receptor, thereby generating a full color image. This process is ideally suited to the output of digitally stored image information. It has the additional benefits of not requiring chemical processing and of not employing materials that are sensitive to normal white light.
As discussed above, laser-induced transfer can involve either mass transfer of the binder, colorant and infrared absorber, giving a bi-level image in which either zero or maximum density is transferred (depending on whether the applied energy exceeds a given threshold), or dye sublimation transfer, giving a continuous tone image (in which the density of the transferred image varies over a significant range with the energy absorbed). Laser-induced mass transfer has been characterized in the literature, in Applied Optics, 9, 2260-2265 (1970), for example, as occurring via two different modes. One mode involves a less energetic mode in which transfer occurs in a fluid state (i.e., by melt transfer), and one mode involves a more energetic mode in which transfer occurs by an explosive force, as a result of generation and rapid expansion of gases at the substrate-coating interface (i.e., by ablation transfer). This distinction has also been recognized in U.S. Pat. No. 5,156,938 (Foley), U.S. Pat. No. 5,171,650 (Ellis), U.S. Pat. No. 5,516,622 (Savini), and U.S. Pat. No. 5,518,861 (Covalaskie), which refer to ablation transfer as a process distinct from melt transfer, and refer to its explosive nature, as opposed to U.S. Pat. No. 5,501,937 (Matsumoto), U.S. Pat. No. 5,401,606 (Reardon), U.S. Pat. No. 5,019,549 (Kellogg), and U.S. Pat. No. 5,580,693 (Nakajima), which refer to transfer of a colorant in a molten or semi-molten (softened) state, with no mention of explosive mechanisms.
Thermal transfer systems have been developed that overcome the disadvantages previously described for the dye transfer systems and mass transfer systems. These systems utilize a mechanism referred to as laser-induced film transfer (LIFT) and multi-LIFT, which is utilized when there is more than one layer of transfer material. Such systems have been reported in U.S. Pat. No. 5,935,758 (Patel et al.) and U.S. patent application Ser. No. 10/461,738 (Kidnie et al.). The LIFT system includes components such as crosslinking agents and bleaching agents to further promote a more controllable dot size and more reproducible and accurate colors. The crosslinking agent reacts with the donor binder upon exposure to infrared laser radiation to form a high molecular weight network. The net effect of this crosslinking is better control of the melt flow phenomena, transfer of more cohesive material to the receptor and higher quality dots. Although other systems involve crosslinking a colorant layer subsequent to transfer to the receptor to prevent back transfer during transfer of the next colorant layer, as in U.S. Pat. No. 5,395,729 (Reardon) and EP 160 395 (ICI) and 160 396 (ICI), the ability to effect crosslinking as a direct result of laser transfer, and hence produce a durable transferred image that is not prone to back transfer represents an improvement over Reardon and ICI.
Using the LIFT or multi-LIFT systems, a half tone image can be formed by the transfer of discrete dots of a film of binder, colorant and additives from the donor element to a receptor element. The dots are formed from a molten or softened film and have well-defined, generally continuous edges that are relatively sharp with respect to density or edge definition; in other words, the dots are formed with relatively uniform thickness over their area. This is in contrast to the dye transfer and mass transfer methods previously described. Dye transfer methods involve transfer of the colorant without the binder and mass transfer methods such as ablation propel fractions of the transfer material but at least partially decomposing the binder. Neither of these methods produces well defined dots with relatively uniform thickness.
Although the LIFT and multi-LIFT systems produce dots having well-defined, generally continuous edges that are relatively sharp with respect to density or edge definition, it has been observed that the thermal absorbing material, such as the cationic infrared absorbing dye, exhibits reduced absorbance as it ages. Because the LIFT and multi-LIFT systems utilize laser energy to effect the transfer of material, the reduced absorbance of the thermal absorbing material has a detrimental impact on overall dot quality. Therefore, there is a need for a thermal transfer system that can produce dots having well-defined, generally continuous edges that are relatively sharp with respect to density or edge definition with greater stability of the thermal absorbing material.